Bonus refused because of resignation given later

In June 2014, after 10 month in the same company in Germany, my employer revised my salary and added a variable part to it. Conditions to receive the totality of this "bonus" variable part have been set up on annual performances. In december my superior confirmed me that I reached all objectives we agreed on. Everything has been written on the performance sheet. At the end of December 2014 I gave my resignation letter since I had to move back to France for personal reasons. Now in January 2015, my manager is telling me that one person from the top management refuses to pay this bonus giving the reason that I am now about to leave the company. However I have now two concerns:

1. I doubt that my resignation has anything to do with the performance of the previous year and hus I doubt that such a reason can be valid and acceptable
2. second the person that my superior mentionned and who - according to him - refuses the payment, is not the person who signed my salary increase and bonus confirmation. I have serious doubts about the legacy of the decision and I would like to know what would be the next steps you would recommend to address the right persons at the right time. Thank you so much in advance!